The present invention is related to a gradient adjusting structure of treadmill, which is able to stably support the treadmill.
A conventional treadmill is generally equipped with a gradient adjusting structure for changing the slope of the tread platform and enhancing exercising effect. FIG. 9 shows an adjustment device for adjusting the height of the frame of the conventional treadmill. The adjustment device includes two linking plates 7 each of which is formed with a frame pivot hole 71, a front support lever pivot hole 72 and a long shaft pivot hole 73. The frame pivot hole 71 and the front support lever pivot hole 72 are respectively positioned at front end and the middle of the linking plate 7 in higher positions. The long shaft pivot hole 73 is positioned at rear end of the linking plate 7 in a lower position. Via the frame pivot holes 71, the linking plates 7 are respectively pivotally disposed on two sides of front end of the tread frame 6. Via the front support lever pivot holes 72, the linking plates 7 are pivotally connected with the front support lever 5. Then, a long shaft rod 731 is passed through the long shaft pivot holes 73 of the linking plates 7. Finally, an adjustment mechanism 8 is pivotally disposed on the long shaft rod 731. The operating end of the adjustment mechanism 8 transversely extends out to the tread frame 6 and is located on the locating plate 51 of the front support lever 5. Accordingly, by means of the operating member of the adjustment mechanism 8, when the drag worm 81 is displaced forward or rearward, the long shaft rod 731 is synchronously moved. At this time, the linking plates 7 will swing about the front support lever pivot hole 72, whereby the height of the position of the flame pivot hole 71 is adjusted to change the support angle of the tread frame 6 so as to ascend/descend the tread frame 6.
The tread frame 6 is pivotally connected with the front support lever 5 via the two linking plates 7. The drag worm 81 of the adjustment mechanism 8 is extended/retracted to ascend/descend the tread frame 6. Accordingly, the weight loaded on the tread frame 6 and the tread force are borne by the linking plates 7. The force exerted onto the linking plates 7 is applied to the drag worm 81 of the adjustment mechanism 8. When the tread frame 6 is descended to a lowest position, the drag worm 81 is extended to a longest extent. Under such circumstance, a bending stress is applied to the drag worm 81 to wear the worm 81. A sleeve 82 is fitted on the drag worm 81 between two locating plates 51 to buffer the force applied to the drag worm 81. However, this is not so helpful to the drag worm 81 with respect to bending stress and even the sleeve 82 will interfere with the rotation of the drag worm 81.
Furthermore, the tread frame 6 is ascended/descended with the front support lever pivot holes 72 of the linking plates 7 serving as the axis. In order to lift the front end of the tread frame 6 to a certain height, the distance between the front support lever pivot hole 72 and the frame pivot hole 71 must have a certain length. Accordingly, the force arm h1 between the front support lever pivot hole 72 and the frame pivot hole 71 of the linking plate 7 will be excessively long. When the tread frame 6 is upward pivotally folded about the frame pivot hole 71, as shown in FIG. 10, the overlong force arm h1 will increase the force applied to the front support lever pivot hole 72 of the linking plate 7. Therefore, after folded, the tread frame 6 has poor support strength and can be hardly stably supported.